vpath.rb: separator for make

* tool/vpath.rb (VPath#list): default separator to PATH_SEPARATOR from
  configure.in for make, not same name constant of File for use in ruby.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@38839 b2dd03c8-39d4-4d8f-98ff-823fe69b080e